This invention relates to an electronic control apparatus for internal combustion engines such as automobile gasoline engines and more particularly to a control apparatus for internal combustion engine having a learning function in order to constantly control the engine under the direction of optimized control parameters.
In an internal combustion engine such as a gasoline engine (hereinafter simply referred to as an engine), it is required that, for example, the supply amount of fuel be kept at a predetermined proportion to intake air and that the ratio therebetween (termed an air/fuel ratio) be always maintained correctly.
Conventionally, the intake air amount is measured and in accordance with measurement results, the supply amount of fuel is controlled such that a predetermined air/fuel ratio can be obtained. This method, however, can not achieve sufficiently accurate controlling from the standpoint of exhaust gas regulation.
Under the circumstances, a so-called air/fuel ratio feedback control method has been employed in which an air/fuel ratio sensor is used to detect an exhaust gas state and feedback-control the supply amount of fuel.
Conveniently, in the air/fuel ratio feedback controlling, the range over which the engine rotation number changes and the range over which the intake air amount changes are respectively divided into, for example, 10 sections and these sections are combined together to define 100 running areas. Area correction coefficients for the respective running areas are initially determined such that the stoichiometric air/fuel ratio equalling 14.7 can be obtained in each of the running areas, and these area correction coefficients are then stored in a memory. During engine running, the coefficients are read out of the memory as necessary so as to be used for calculating the injection amount, thus permitting the individual running areas to take the stoichiometric air/fuel ratio. This expedient can therefore prevent a transient aggravation of exhaust gas due to a delayed response in the air/fuel ratio feedback.
Incidentally, control characteristics for the engine greatly differ from one engine to another, depending on irregularity of characteristics of the engine per se and irregularity of characteristics of various sensors and actuators used for controlling.
This means that if the area correction coefficients necessary for the area correction method are initially prepared in view of a standard engine and applied to all of the other engines, almost no fruitful results can be obtained. Therefore, different sets of area correction coefficients must be prepared independently for different engines and then must be stored in ROM's respectively dedicated to the different engines, thus giving rise to degraded productivity prone to high costs.
Further, since the characteristics of the engine per se change with time and the characteristics of the sensors and actuators also change with time, it is frequent that area correction coefficients which are set at the initial phase of manufacture become almost unmatched as time elapses.
As a countermeasure for the above problems, a learning control scheme has recently been highlighted in which a memory capable of being written or rewritten with data is used for storing area correction coefficients and running areas (learning areas) of the memory are sequentially written and supplemented or rewritten with area correction coefficients experiencing learning during engine running, whereby accurate area correction coefficients (learn correction coefficients) based on the latest results of running can constantly be prepared for air/fuel ratio controlling.
According to the learning control scheme, the area correction coefficient need not be prepared initially and besides when characteristics of, for example, the engine change, the area correction coefficients can be self-corrected correspondingly, thereby ensuring that constantly correct controlling can be expected and the aggravation of exhaust gas, inclusive of the transient aggravation, can be prevented.
A prior art apparatus based on the learning control scheme is known as disclosed in, for example, JP-A-60-90944. In the known apparatus, a difference between an air/fuel ratio correction coefficient produced from an air/fuel ratio sensor and a reference value is determined and a learning value of a learning correction term used for correcting the air/fuel ratio correction coefficient is renewed by adding the difference at a predetermined proportion or percentage.
When as in the prior art the learning value of the learning correction term used for correcting the air/fuel ratio correction coefficient is renewed adding the difference between air/fuel ratio correction coefficient and reference value at the predetermined proportion, the renewed learning value becomes overestimated for a value of the difference or underestimated for another value of the difference, indicating that proper learning can not be undertaken. Another problem encountered in the prior art is that after a particular learning area is renewed, renewal of learning areas adjacent to the particular learning area is not done for an interval of time in the event that a certain learning condition is imposed, with the result that controlling can not be shifted smoothly from one learning area to another.